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It's been really cold, with lots of snow. So what happened to global warming?

Temperature anomalies (variations above and below average, in degrees Celsius) for Wednesday, January 17, 2007. Despite the cold across Texas and New England, most of the Northern Hemisphere was running well above average that day. Click here or on the image to enlarge. (Image courtesy NOAA/ESRL PSD Map Room.)

Climate change doesn't mean that winters will disappear or that summers will be uniformly hot. There are always cold spells and warm spells going on in one place or another. But even where weather is cold, what's considered "typical" can change. For example, the heavy snow that struck Colorado and Kansas at the end of 2006 was actually more characteristic of that area's autumn or spring weather than a typical December. Time will tell whether and where such individual cases recur often enough to be considered a trend.

To examine long-term warming, climate scientists often look at larger areas and longer time periods. Globally, Earth's natural processes don't follow a linear pattern, so the global average temperature may be slightly cooler or warmer from one year to the next. Different parts of Earth's ecosystem also respond to the greenhouse effect in different ways. The oceans, for example, hold more heat and respond to atmospheric changes more slowly than land masses do. Average temperatures of the land, oceans, and atmosphere also vary from year to year as well as from each other.

Just as a baseball player on steroids will occasionally strike out, a climate warmed by extra greenhouse gases will still produce unusually cold weather at times. It's also important to remember that there are multiple factors contributing to every weather event—which is why you'll often hear forecasters and researchers pointing out that no particular weather feature can be entirely "blamed" on climate change.

To learn more about the links between weather events and global climate change, see our in-depth report Weather on Steroids.

National Center for Atmospheric Research | University Corporation for Atmospheric Research